A liquid crystal display device is a display device which is used for a notebook computer, a personal computer, a smart phone, or a television and a characteristic thereof is improved every year in accordance with enlargement of a demand of the liquid crystal display device.
A liquid crystal panel of a liquid crystal display device which is a non-emitting element requires a backlight unit due to its structure. The backlight unit is configured by various optical systems. Further, the backlight unit uses optical films which are periodically arranged in order to improve brightness.
FIG. 1 is a view schematically illustrating a configuration of a liquid crystal display device of the related art.
As illustrated in FIG. 1, a backlight unit 10 includes a light emitting source 1, a reflecting plate 2, a light guide plate 3, a diffuser sheet 4, a first optical sheet 5, a second optical sheet 6, and a protecting sheet 7.
The light emitting source 1 is an element which generates visible rays and as for the light emitting source 1, a light emitting diode (LED) and a cold cathode fluorescent lamp (CCFL) may be selectively used.
The light emitted from the light emitting source 1 is incident onto the light guide plate 3 to be progressed while being totally reflected inside the light guide plate 3 and light which is incident onto a surface in the light guide plate 3 at an incident angle which is smaller than a threshold angle is not totally reflected but transmitted to be emitted to an upper side and a lower side.
In this case, the reflecting plate 2 reflects the light which is emitted to the lower side to be re-incident onto the light guide plate 3 to improve optical efficiency.
The diffuser sheet 4 diffuses the light emitted through an upper surface of the light guide plate 3 to uniformize brightness and broaden a viewing angle, so that the light which passes through the diffuser sheet 4 has reduced front emission brightness.
The first optical sheet 5 is configured by a base member 5b and a structural pattern 5a to primarily concentrate and emit light which enters from the diffuser sheet 4 so as to be refracted and vertically incident thereonto.
Further, the structural pattern 5a is integrally formed on an upper surface of the base member 5b to vertically refract and emit light which is incident through the base member 5b. 
The structural pattern 5a is generally formed to have a triangular cross-section and a vertical angle of the triangular shape is generally approximately 90 degrees.
The second optical sheet 6 has the same shape as the first optical sheet 5 and secondarily concentrates and emits the light which is primarily concentrated by the first optical sheet 5 in order to increase brightness thereof.
Here, the first optical sheet 5 and the second optical sheet 6 are integrally attached such that an extending direction of the structural pattern of the first optical sheet 5 and an extending direction of the structural pattern of the second optical sheet 6 perpendicularly intersect, thereby increasing the brightness.
The protecting sheet 7 is attached onto an upper surface so as to prevent surface damage to the second optical sheet 6.
However, according to the above structure, when the first optical sheet 5 and the second optical sheet 6 are bonded, an upper end of the structural pattern 5a is bonded to a lower surface of the second optical sheet 6, so that a shape is modified and a length of a cross-sectional trace is reduced and thus a region where light which is transmitted from a lower portion is actually refracted and concentrated is undesirably reduced.
As described above, when the region of the structural pattern 5a where the light is refracted and concentrated is reduced, brightness is reduced and thus a quality of the backlight unit is lowered.